import pandas as pd
# pd.set_option['display.unicode.east_asian_width',True]
df=pd.DataFrame({'year':['2019','2020','NaN','2020','2022'],
                 'month':['3','NaN','6','9','12'],
                 'day':['4','2','21','2','15']})
print(df)
print(df.fillna({'year':2021,'month':9}))